Modern Machine-Shop Practice Part 157

[Ill.u.s.tration: Fig. 2337.]

[Ill.u.s.tration: Fig. 2338.]

[Ill.u.s.tration: Fig. 2339.]

[Ill.u.s.tration: Fig. 2340.]

[Ill.u.s.tration: Fig. 2341.]

[Ill.u.s.tration: Fig. 2342.]

It will be noticed in Fig. 2331 that the bra.s.s B does not fill the s.p.a.ce in the rod. This is because that bra.s.s has to pa.s.s in over crank-pin collar and push up into the journal after it is in the rod. To make this s.p.a.ce as small as possible, and to enable giving the crank pin as large a collar as possible, the key bra.s.s is sometimes beveled off, as shown in Fig. 2336 at A B. Another form of this rod end is shown in Fig. 2336, in which there are two keys to the bra.s.ses, the object being to adjust the keys to maintain the rod of its proper length. In order to facilitate making this adjustment, there should always be upon the face of the rod end centrepunch marks, as shown in Fig. 2338 at F and G, or else two deep marks, as shown at C D in Fig. 2337. Then, in lining up the bra.s.ses to set the key back, the rod may be restored to its original length by putting behind the back bra.s.s a piece of metal of such thickness as will bring the centre of the bore of the back bra.s.s B even with the centrepunch or other marks. This being the case, it does not matter about the exact thickness of the piece of metal put behind the other bra.s.s, since a variation in that will only act to let the key come more or less through the rod end without affecting the length of the rod. In Fig. 2337 is shown a form of rod end sometimes used. The end being open, the bra.s.ses pa.s.s through it. In this case the whole strain of the pull of the rod falls upon the edge of the gib at top and bottom of the strap, causing the gib to wear out very fast; furthermore, the back bra.s.s condenses the metal at the back of the bra.s.s opening, acting to pene it and throw the points of the rod end open, which it always does, the jaws of the gib imbedding in the jaws of the rod. This opening of the rod jaws makes the bra.s.ses loose in their places; hence this is a weak and undesirable form of rod end, though very convenient to take on and off. In Figs. 2338, 2339, 2340, and 2341 is shown a form of solid-ended rod of more modern construction. In this case a wedge A is used instead of a key, being adjusted by screws pa.s.sing through the rod at the top and bottom, it being obvious that the set-screws may have check-nuts added. B is the back bra.s.s, and C the key bra.s.s. In this case the f.l.a.n.g.e of the bra.s.s goes next to the crank pin, and a plate D is provided to serve as a f.l.a.n.g.e on the front face of the bra.s.s. In Fig.

2338 this plate is removed to show the wedge A; but it is shown in the plan view, 2339, and the end view, 2340, and by itself in Fig. 2341. A groove is cut on each side of the two bra.s.ses and the plate spans the bra.s.ses, pa.s.sing up the groove being held in position by a screw at E.

The opening for the bra.s.s (in the rod end) is here shown wide enough for the rod end to pa.s.s over the collar of the crank pin, but in many cases, with this as well as with other forms of solid-ended rods, the crank pin may be made plain--that is, without a f.l.a.n.g.e--and have a washer secured by a screw, so that by removing the washer the rod may be put on with the bra.s.ses already in place, and made no thicker (at the joint face) than is necessary for strength. In Fig. 2342 is shown what may be termed a clip-end connecting rod, the screw closing the rod end (to take up the wear) against the spring of the metal. It is obvious that in this case the hole may receive a bra.s.s bush split as is the rod end and secured from turning by a pin. Fig. 2343 presents another form of solid-end rod, which admits of the use of a bra.s.s having a f.l.a.n.g.e on both sides of the strap, and will take on and off by removing the cap B. If the crank-pin collar is solid, the bra.s.ses must be placed on the crank pin, and the rod, with the wedge in place, lifted or lowered to the bra.s.ses; but if the crank pin has a washer and bolt, the rod may be put together and slipped on its place.

A compromise between the solid and the strap-rod end is shown in Fig.

2344, which represents a design used upon the fast engines of the Pennsylvania Railway. The piece A takes out to enable putting on the rod or taking it off, A being secured in position by the bolt and nuts shown. This forms a solid and durable rod that is much less costly to make than strap-ended rods.

[Ill.u.s.tration: Fig. 2343.]

[Ill.u.s.tration: Fig. 2344.]

[Ill.u.s.tration: Fig. 2345.]

[Ill.u.s.tration: Fig. 2346.]

[Ill.u.s.tration: Fig. 2347.]

[Ill.u.s.tration: Fig. 2348.]

The simplest form of strap-ended connecting rod is that shown in Fig.

2345; S is the strap, secured to the rod end by the key D and gib C. A is the top, and B the bottom, or crown bra.s.s, and E the set-screw for securing the key in its place. [When the rod ends are forged in separate pieces, to be afterwards welded to the stem of the rod after the strap bra.s.ses are fitted up (which is done for convenience in handling them while fitting them up), they are termed stub ends.] This form of rod affords great facility for connection with the journals as the strap is easily removed. As the strap, however, is only secured to the rod by the gib and key, and as these have a small amount of area on the sides, it is not unusual to employ two gibs and one key, as in Fig. 2346, which holds the strap more securely, and more effectually prevents its movement sideways upon the rod end. In rods in which gibs and keys alone are used to hold the strap to the rod, the strap moves along the rod as the key pa.s.ses farther through the strap, and the fit of the strap to the rod must be easy enough to permit of this motion; hence it cannot be locked to the rod. This, however, may be done by the employment of a bolt as well as a gib and key, as is shown in Fig. 2347. The edge of the gib here abuts against the back of the top bra.s.s, or key bra.s.s, as it is sometimes termed, which is objectionable, inasmuch as that it is apt to indent the bra.s.s, as shown in Fig. 2348 at B. This causes the bore to close at A, and causes the journal to heat, while it makes the bra.s.s fit loosely between the jaws of the strap, because it stretches the metal at the back of the bra.s.s, which has the same effect as pening it with the hammer.

In Fig. 2349 is shown an end of a connecting rod, such as is employed on American locomotives, the use of a gib being dispensed with, and the strap being held by two bolts. To prevent the edge of the key from imbedding in the bra.s.s, a piece of hardened steel is sometimes placed between the key and the bra.s.s, as shown in the figure.

In some designs this method is reversed, the gib being prolonged in a screw-thread, as shown in Fig. 2350, and the key head is carried over as shown. Two wing nuts are provided for adjusting the key, which enables its adjustment without the employment of a wrench or hammer.

To prevent the end of the set-screw from raising a burr on the key, which would prevent its easy motion through the keyway, a shallow groove is sometimes cut along the key, as in Fig. 2351 at A, the end of the set-screw binding on the bottom of that groove.

In other forms of rod a gib and key are used as well as two bolts. This not only holds the strap very firmly, but it prevents to a certain extent the pening of the back of the bra.s.s, explained with reference to Fig. 2348. It is obvious that in the absence of a gib the key moving under friction against the bra.s.s stretches the metal more than a gib that presses against the bra.s.s, but has no motion endways.

[Ill.u.s.tration: Fig. 2350.]

[Ill.u.s.tration: Fig. 2351.]

[Ill.u.s.tration: Fig. 2352.]

In Fig. 2352 the strap is held by bolts having nuts at each end, instead of a solid head at one end and nuts at the other. The single nuts at the top serve to draw the bolts out when the rod is to be taken apart, thus saving the use of the hammer for that purpose.

[Ill.u.s.tration: Fig. 2353.]

In Fig. 2353 is shown a form of rod in which the strap is held by two dies A B, and a bolt which pa.s.ses through the strap, the dies, and the rod end.

[Ill.u.s.tration: Fig. 2354.]

In Fig. 2354 is a form of rod end in which the strap ends are keyed against abutments on the rod by means of the key A. The abutments and strap ends being bevelled, keying up the strap with A closes it down upon the rod.

[Ill.u.s.tration: Fig. 2355.]

In Fig. 2355 is a form of rod end largely used upon marine engine work; A is the end of the rod, B, B the bra.s.ses, and D, D bolts pa.s.sing through the bra.s.ses. Here we have no means of correcting the alteration of length due to the wear, unless a line is marked on the rod end, as at C, and the distance that line should stand from the centre of the bra.s.s bore is marked beside it, as is denoted by the figure in the cut, indicating that the line should stand 9 inches from the cuts of the bra.s.s bore.

In general practice the inside jaw faces of connecting rod straps and the faces of the rod are made parallel, which serves very well when the duty and wear is not great; but when the wear and tear is great, as in locomotive work, it is much better to make them taper; indeed, they are in any event better taper, because in that case the bra.s.ses can be made a tighter fit. The reason for making them parallel is because they can be more readily planed so than taper; but a parallel strap is more difficult to fit, and cannot be made so good a fit as a taper one, even when new, while it is very much more difficult and expensive to repair.

[Ill.u.s.tration: Fig. 2356.]

When the faces of the stub end (or, more properly speaking, of the block) are parallel one to the other, and the inside faces of the strap are also parallel, the strap must be made a very easy fit to the block, in order to be an equal fit from end to end; for if the strap fits as tightly as it should to be a good job, it will, when put on the rod, spring open, fitting across A, Fig. 2356, only; this because the strap springs open from contact at A. The fit, then, can only be such as will not have force enough to spring the strap open, and this is very small indeed even in a very strong strap. It is within the mark to state that in a strap measuring 4 inches between jaws, at A in Fig. 2356, it can be forced by hand on the rod sufficiently tight to spring them open 1/16th of an inch at B, B. When the bra.s.ses are fitted into the strap a second difficulty arises, inasmuch as they must be made a very easy fit, or else they will spring the strap open so that it will neither fit at A nor at B, whereas it is desirable that the bottom bra.s.s drive home, and the top bra.s.s, or one nearest the rod, just push home by hand.

When the rod requires repairing a more serious difficulty arises.

Suppose, for example, that the strap requires refitting to the rod, then it must evidently be closed between the jaws, especially if the rod end requires filing up, as it usually does. Now the jaws being parallel cannot be closed without being taken to the blacksmith shop and closed across the crown, as at A in Fig. 2357; for if the jaws are closed (as they might be) by pening the corners B, C the jaws would close as denoted by the dotted lines. The bra.s.ses will have to be made larger than the diameter at D, in order to fill the s.p.a.ce at A, and will be an easier fit as they pa.s.s from D to A, whereas the opposite should be the case. The strap must therefore be closed across A in the blacksmith's fire; this will scale the crown end and render it necessary to file down the whole of the surface on each of the side faces of the strap and rod in order to make them parallel, as they must be to have the f.l.a.n.g.es of the bra.s.ses fit when home in the strap.

[Ill.u.s.tration: Fig. 2357.]

The blacksmithing will in most cases render it necessary to file out the keyways, and this again entails the making of a new gib and key. All this extra work may be avoided by making the block and strap a little taper. But before proving this it may be noted that when the rod is made parallel the strap may be made to fit tightly by making the jaws taper, as shown by dotted lines in Fig. 2357; so that when the strap is on the rod, and the jaws spring open by reason of the close fit, the fitting surfaces will be parallel. Such a construction would be faulty however, for the bra.s.ses would fit too tight when entering the strap, and get easier as they pa.s.sed to their places, whereas, as already stated, the exact opposite should be the case.

[Ill.u.s.tration: Fig. 2358.]

Let us now observe the advantages of a strap, whose inside faces are made as in Fig. 2358; smaller at A than at B, and also at C than at D, while the thickness from A to B is greater than that from C to D, while the widths C D are less than the corresponding width of the rod.

First, as to fitting the strap to the rod. It may be made so tight to the rod that it will only just pa.s.s on when pushed by the hand.

Second, this will render possible a tighter fit than would be possible with a parallel strap and rod.

Third, the width B A being taper, the bra.s.ses may be easier made a good fit, because there will be some metal to fit on after they enter at B.

Fourth, the bra.s.ses may be made a tighter fit, the bottom bra.s.s being tight enough to spring the strap a trifle, easing but not destroying its fit on the rod.

Fifth, the top bra.s.s may be made a handsliding fit to the strap without springing the strap open, which being already under a tension because of the spring due to the bottom bra.s.s, will be more rigid and permit of a tighter degree of top bra.s.s fit, without springing open and away from the rod.

Sixth, this will leave the bottom bra.s.s a tight driving fit, and the top a hand sliding fit, which is desirable, because the top bra.s.s has to be taken out to get the rod off while the bottom bra.s.s remains in its place.

[Ill.u.s.tration: Fig. 2359.]

Seventh, what is of more consequence than all, the strap can be more easily and cheaply refitted to repair it. Thus, in Fig. 2359, suppose the strap to have been closed by pening at D; then whether the end D will be narrowed will depend on the amount the strap was closed, and the amount of taper it had before closing. Let us take, however, the most unfavorable conditions, and suppose that the amount of taper was so small, and the amount of closing by pening so great, that the jaws were made taper and smallest at D. Then the amount to be filed off to bring the width of jaw correct, and a fit to the strap, will be less than if the strap jaws were formed as in Fig. 2357, as will be seen by comparing Fig. 2357 with Fig. 2359, the amount to be filed away being that between the dotted and the full lines in both figures; the amount of closure being the same in the two figures.

But there is another great advantage, inasmuch as in pening, the strap may be pened and tried on the rod, the strap being pened and tried alternately until the required fit is obtained, which is not practicable with upsetting in the blacksmith's shop.

Again, the keyways in the strap will not be set out of true with those in the rod, as they are apt to be when upsetting is resorted to, nor will the strap be scaled; hence the side faces will require but little filing.